Professional diver Scott Gardner was just about out of air and swimming back to the surface when he heard an odd cracking sound nearby. Swimming over to investigate, he spotted the foot-long (30-centimeter-long) fish at work.

"When Scott showed me his photos, I said 'Wow, this is quite amazing,'" said study co-author Alison Jones, a coral ecologist at Central Queensland University in Rockhampton, Australia.

—Ker Than

Photograph courtesy Scott Gardner

Putting Its Whole Body Into It

The tool-using tuskfish turns sideways to gain leverage as it bashes the cockle against the rock.

Not everyone agrees the shellfish-smashing fish is actually using a tool. "It's taken six years to publish this research, because reviewers argued back and forth about whether it was a true example of tool use or not," co-author Jones said.

At issue is whether the tuskfish behavior fits the classic definition of tool use, which requires an animal to actually hold or carry the tool and use it to manipulate another object.

But that definition was based on human and primate behavior, says Jones and her team, and can't be applied to fish.

"We argue that tuskfish have a different type of intelligence that's evolved for their specific environment. ... They don't have hands, and they have resistance in the water, so you can't expect them to use a tool the way a primate would."

After opening the cockle, the tuskfish bares the teeth that gave it its name.

If scientists accept that tuskfish are indeed capable tool users, then it raises all sorts of interesting questions about fish behavior, the team says.

For example, do the tuskfish use the same spot each time to crack open shellfish? If they do, it "adds weight to the long-held view that fish have a cognitive map and they can remember where to go," Jones said.

It may be a while before the team has an answer. The behavior hasn't been seen since 2006, but the researchers hope to catch a tuskfish in the act again soon.

"We're hoping to set up a camera at one of these sites at some point and get more information, but it's technically demanding," Jones said.

Cracked and empty shells lie in heaps on the seafloor at the site of the smashing incident.

If tuskfish can use tools in the wild, then it's possible other fish species can too—and that could have broader implications for fisheries management, Jones said.

"If older fish are teaching younger fish these patterns of behavior, then by taking out all of the older"—and therefore bigger and more attractive to fishers—"fish, will the younger fish be less able to forage effectively?" Jones said. "That's one of the critical questions that this observation raises."